The present invention relates to an air/fuel ratio control system for an internal combustion engine where air flow and exhaust gas recirculation flow are calculated from pressure sensors.
Engine control systems often determine the amount of fuel to inject by measuring a manifold pressure, along with other engine operating conditions. This method is often referred to by those skilled in the art as the speed density method. In this method, a mean value model of engine operation is constructed, where an average manifold pressure at a given speed results in a certain air flow into the cylinder. In this type of system, measurement of the manifold pressure is critical for proper prediction of the air flow into the cylinder and thus for proper air/fuel ratio control.
As stated above, many methods are available to estimate cylinder air charge using a manifold pressure sensor. Typically, engine maps are provided that provide a cylinder air charge as a function of measured manifold pressure, manifold temperature, and engine speed. In engines that also utilize exhaust gas recirculation, an improved cylinder air charge estimate is obtained by providing adjustments based on the amount of exhaust gas recirculation.
One particular method is described in U.S. Pat. No. 5,205,260. In this method, an EGR flow is estimated based on differential pressure across a flow control valve and based on a cross-sectional area of the valve. Then, this flow is used in a manifold filling model to estimate a partial pressure of EGR in the intake manifold. Then, based on this partial pressure of EGR and measured manifold pressure, a cylinder air charge value is computed.
The inventors herein have recognized a disadvantage with the above system. In particular, estimating EGR flow in this manner leads to estimation inaccuracies, due to valve area uncertainty. This uncertainty may be caused by deposits in the valve. Since inaccuracies in EGR flow directly affect estimated cylinder air charge, this leads to inaccuracies in calculated fuel injection amount and therefore may degrade air-fuel ratio control.
Another approach to determining EGR amount has used a differential pressure measurement across an orifice to infer a flow of exhaust gas. Traditionally, the orifice is located upstream of the exhaust gas recirculation flow control valve. Thus, the pressure measurements are shielded from the intake manifold pressure pulsations; however, the pressure measurements are not shielded from the exhaust pressure pulsations. In the traditional system, the high frequency pressure pulsations present in the pressure measurements are reduced by using a conventional low pass filter. Such a system is disclosed in U.S. Pat. No. 5,613,479.
The inventors herein have recognized a significant opportunity to reduce total system cost by relocating the orifice downstream of the exhaust gas recirculation flow control valve but before the intake manifold. Thus, the manifold pressure sensor can be used to measure the pressure downstream of the orifice and a single absolute pressure sensor can be used to measure the pressure upstream of the orifice. This creates the needed differential pressure to measure exhaust gas recirculation flow, as well as an opportunity to estimate cylinder fresh charge.
An object of the invention claimed herein is to provide an exhaust gas recirculation measurement system and cylinder air charge estimation system with improve accuracy.
The above object is achieved, and problems of prior approaches overcome, by a system for estimating engine flows, including exhaust gas flow from an exhaust manifold of an internal combustion engine to an intake manifold of the engine, the system comprising: a flow control valve having a variable orifice positioned in an exhaust gas recirculation path between the exhaust manifold and intake manifold of the engine; a fixed orifice area located in said path and downstream of said valve; and a computer for measuring a first pressure between said fixed orifice area and said flow control valve, measuring a second pressure downstream of said fixed orifice area, calculating a recirculated exhaust flow based on said first pressure and said second pressure, and determining an air amount based on said second pressure and said calculated recirculated exhaust flow.
By using common signals for estimating EGR flow and cylinder air charge, a simplified structure and reduced cost system is obtained. Further, by using a fixed orifice area downstream of a control valve to recirculate exhaust gas along with pressure measurements upstream and downstream of the orifice, a more accurate EGR flow estimate is obtained. In particular, the estimation scheme does not have to account for the changing valve area, and thus less affects have to be included. Further yet, the present invention does not need to measure or infer exhaust manifold temperature or exhaust manifold pressure.
An advantage of the above aspect of the invention is that more accurate cylinder air charge estimate is obtained.
Another advantage of the above aspect of the invention is that the more accurate cylinder air estimate yields improved emissions at a reduced system cost.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.